Computing devices store ever-increasing amounts of digital data, such as documents, pictures, music, and movies. Digital data is increasingly being stored in “the cloud,” that is, stored in distributed data sources accessible over a ubiquitous network like the Internet. For example, some cloud services provide general-purpose networked data storage. Other cloud services provide networked access to specialized data storage, such as music libraries. Data stored by cloud services may be private (e.g., accessible only to a data owner), may be public, or may have an intermediate level of allowed access. A data owner may maintain several such cloud data stores across several different cloud service providers. Each cloud service provider generally has its own authentication system and security model.
While networked data sources are increasingly common, local storage devices like memory cards and disk drives continue to store much—or most—digital data. Data stored in local storage devices is generally not accessible beyond a local network.